Experiments already in progress upon buffers, especially sodium beta-glycerophosphate, as substitutes for sodium bicarbonate; upon the role of Na/K ratio; and upon substitutes for the heat labile compound L-glutamine, will be completed and data upon the growth, differentiation and function of several cell types, including some hormone-dependent epithelial cells, in autoclavable defined culture media incorporating these innovations, will be assembled. The effects of antibiotics, usually considered innocuous in culture media containing serum, as selective inhibitors of the growth and attachment of particular cell types in serum-free media, will continue to be examined. The use of D-valine and cis-hydroxyproline for selection against fibroblastic and for epithelial cells, in cultures in defined media, will be evaluated. An analysis of the kinetics of lysis of mature and immature mouse erythrocytes by NH4Cl, with the objective of adding precision to NH4Cl lysis as a means for removing erythrocytes from cells for primary cultures with minimal effects upon the viability of the cells in culture, will be completed.